Interferons (IFNs) are multifunctional cytokines that play a critical role in innate immunity. Type I IFNs (IFNs-1, -2, -I and others) exhibit immunomodulatory, antiproliferative and antiviral activities, although gene knockout experiments have suggested that, among these effects, the antiviral function is most critical for survival. IFNs elicit their effects through the induction of discreet subsets of early response genes, known collectively as IFN-stimulated genes (ISGs). To date, however, the precise functions of many individual ISGs remain poorly understood. We have performed extensive gene expression profiling of genes regulated by type I IFNs in a variety of normal and malignant human cell types. These studies have highlighted large repertoires of both common (e.g. induced in all cell types) and unique (cell-specific) ISGs. Here, we propose to assess the physiological function of the protein encoded by the ISG12a gene, an IFN-induced gene that was implicated in our gene array studies as an ISG strongly induced in all cell types. Our recent studies have shown that the ISG12a protein is localized to the mitochondrial membrane, and that ectopic ISG12a expression results in a sensitization of cells to other apoptotic stimuli. Since apoptotic responses to virus are an important component of an overall IFN response, we hypothesize that ISG12a influences the ability of cells to respond to viral infections by impacting mitochondrial function, and possibly regulating apoptotic responses. Thus, we propose to evaluate the physiological importance of endogenous ISG12a in both IFN-dependent mitochondrial responses and in antiviral effects by conducting exploratory studies that encompass these specific aims: Aim 1. To determine the significance of ISG12a in IFN-induced cellular and mitochondrial changes Aim 2. To determine the significance of ISG12a induction on antiviral and other innate immune responses PUBLIC HEALTH RELEVANCE: The ability of the body to respond to invading organisms, including virus and bacteria, depends on the coordinated actions of both immune cells and other cell types in the body. Production of interferon by virally infected cells upregulates the body's ability to sense and respond not only to other viruses, but also to other invading microorganisms. We have identified an interferon-induced protein, called ISG12a, that we feel is involved in this sensitization process, and better understanding of ISG12a's effects on cells should lead to improved understanding of how the body regulates responses to virus and other pathogens.